The present invention relates to coated sodium percarbonate particles consisting of a core of sodium percarbonate produced in the aqueous phase by crystallization and a firmly adhering, two-layer coating surrounding the core. The sodium percarbonate particles coated according to the invention by means of a fluidized bed spray process are distinguished by high active oxygen stability in detergents and also by high abrasion resistance and good silo storage properties. The invention further relates to the use of the coated sodium percarbonate particles in detergents, cleaning agents and bleaches.
Sodium percarbonate (2 Na.sub.2 CO.sub.3 .multidot.3 H.sub.2 O.sub.2), which is obtainable both by crystallization in the aqueous phase--see for example DE-PS 28 00 760--and by fluidized bed spray processes--see for example DE-A 43 29 205 --, is used as an active oxygen component in detergents, bleaches and cleaning agents. Owing to the inadequate storage stability of sodium percarbonate in a warm, damp environment and also in the presence of various detergent and cleaning agent components, sodium percarbonate has to be stabilized against loss of active oxygen (Oa). One essential stabilizing technique consists of surrounding the sodium percarbonate particles with a coating consisting of components having a stabilizing action.
It is known from EP-A 0 654 440 to increase the abrasion resistance and, at the same time, the storage stability of sodium percarbonate produced by crystallization by coating it with sodium percarbonate in a fluidized bed. Coating takes place by spraying the sodium percarbonate particles with an aqueous solution substantially containing sodium percarbonate or, at the same time with an aqueous solution substantially containing soda and one substantially containing hydrogen peroxide, wherein soda (sodium carbonate) and H.sub.2 O.sub.2 substantially being used in a molar ratio of 2 to 3, in a fluidized bed, maintaining a fluidized bed temperature in the range of 40 to 95.degree. C., with water being evaporated. Although the substance data of the sodium percarbonate coated with sodium percarbonate in a fluidized bed, having a core consisting of crystallized sodium percarbonate, are much improved compared with pure crystals, a further increase in active oxygen stability in detergents and cleaning agents would be of interest.
Various processes for increasing stability by coating are known:
GP Patent 174 891 teaches the stabilizing of peroxygen compounds by coating with water glass. However, it is not possible to achieve adequate stabilizing for sodium percarbonate by this method. In addition, the suitability for storing in silos in considerably increased by coating with coating substances containing water glass (because of the reduced caking tendency). With reference to the coating of soda containing water of crystallization, this document also mentions Glauber salt, i.e. sodium sulphate decahydrate, as a possible coating component. However, the coating of sodium percarbonate with a layer of sodium sulphate is not suggested by this document.
In DE-OS 24 17 572 a sodium percarbonate coated in a fluidized bed is disclosed, the coating substance being a mixed compound formed by crystallization of sodium carbonate or bicarbonate with sodium sulphate. According to DE Patent 26 22 610, the single-layer coating contains a sodium silicate in addition to sodium sulphate and sodium carbonate. Despite improved stability of the sodium percarbonate particles coated in this way, the active oxygen content still decreases too strongly during prolonged storage in the presence of a detergent powder. In addition it was found that coated sodium percarbonate particles with soda in the outermost layer display unsatisfactory silo storage behavior, and thus poor flow properties, after prolonged storage.
According to DE-OS 43 15 380, the coating material of a coated sodium percarbonate, the core of which is a sodium percarbonate crystal, consists of a mineral salt mixture substantially of alkali metal sulphate and alkali metal chloride. No suggestion of using sodium sulphate as the sole coating material can be taken from the is document.
A coating according to U.S. Pat. No. 4,325,933, containing exclusively magnesium salts, such as magnesium sulphate, no longer meets current active oxygen stability requirements. Accordingly, WO 95/02555 and EP-A 0 623 553 relate to coated sodium percarbonate particles, the coating of which, in addition to magnesium sulphate and a silicate, also contains soda or an alkali metal slat from the series of carbonates, bicarbonates and sulphates. The single- or multi-layer coating does lead to very good active oxygen stability but the need to use three different coating components is a disadvantage.
WO 95/15291 teaches a process for the production of sodium percarbonate particles with improved stability wherein sodium percarbonate is brought into contact with a CO.sub.2 -rich gas and moisture in a fluidized bed in order to form a continuous sodium bicarbonate layer on the sodium percarbonate particles. A layer of sodium sulphate may additionally be applied on to the sodium bicarbonate layer.
An object of the present invention is to provide other coated sodium percarbonate particles which display very good active oxygen stability in detergents, bleaches and cleaning agents and good silo storage behavior. Furthermore, the required combination of substance properties should be obtainable with a coating which reduces the active oxygen content to the last possible extent.